I just gave an introductory talk to Clojure and one of the questions after the event was when would a Java programmer might want to switch to using Clojure?
Well Java is a much more complex language then Clojure and requires a lot of expert knowledge to use properly. You really have to know Effective Java pretty well, Java still contains every wrinkle and crease from version 1 onwards. By comparison Clojure is a simpler and more consistent language with less chance of shooting yourself in the foot. However as a new language Clojure does not have the same number of tutorials, faqs and Stack Overflow answers. It also has a different structure to curly brace languages, so it feels quite different to program than Java. If you are proficient Java programmer and you are adept with the standard build tools and IDEs then why would you consider changing?
One example is definitely concurrency, even if you were going to do Java then you’re probably going to let Doug Lea handle the details via java.util.concurrent. However Doug Lea didn’t get to rewrite the whole of Java to be concurrent. In Clojure you are letting Rich Hickey handle your concurrency and the whole language is designed around immutability and sensible ways of sharing state.
Another is implementing algorithms or mathematical programming. A lot of mathematical functions are easy to translate into LISP expressions and Clojure supports variable arity functions for recursion and stackless recursion via the recur function. In Java you either end up using poor-man’s functions via static class methods or modelling the expression of the rule into objects which is a kind of context disconnect.
Similarly data processing and transformation work really well in Clojure (as you might expect of a list processing language!), if you want to take some source of data, read it, normalise it, apply some transform functions, perhaps do some filtering, selection or aggregation then you are going to find a lot of support for the common data functions in Clojure’s sequence library. Only Java 8 has support for lambda function application to collections and even then it has a more complex story that Clojure for chaining those applications together to a stream of data.
You might also find Clojure’s lazy sequences helpful for dealing with streams based on large quantities of data. Totally Lazy offers a port of a lot of Clojure’s functionality to Java but it is often easier to just go direct to the source than try and jury-rig a series of ports together to recreate the same functionality.
A final point to consider is how well your Java code is working out currently, if you are having a lot of problems with memory leaks, full GC and so on then it might be easier to work with Clojure than put in the effort to find what pieces of your code are causing the problems. Not that Clojure is a silver bullet but if you use side-effect free functional programming your problems are going to be limited to the execution path of just the function. In terms of maintaining clear code and reasoning about what is happening at run time you may find it a lot easier. Again this isn’t unique to Clojure but Clojure makes it easier to write good code along these lines.